1. Field of the Invention
This invention relates to a piston for internal combustion engines, and more particularly to a piston for internal combustion engines which has a combustion chamber in a top surface thereof.
2. Description of the Related Art
A piston for internal combustion engines which has a combustion chamber in a top surface thereof, for example, a piston for diesel engines which has a re-entrant type combustion chamber (a combustion chamber having an opened portion of a diameter smaller than a maximum inner diameter of the combustion chamber, and a comparatively deep recess) in a top surface thereof is formed by providing an outer circumferential wall of the combustion chamber with a cooling cavity, into which a cooling oil is injected during an operation of the engine to cool a top portion, the temperature of which becomes high, of the piston, especially, a cross-sectionally angular portion (edge portion) defined by an upper edge of an inner circumferential surface at which the combustion chamber is opened in the interior of a cylinder of the combustion chamber and a piston-top surface, whereby an increase in the temperature of the combustion chamber is minimized.
Such a structure is effective when an engine output is at a normal level but, when the level of an engine output is increased high, a high thermal load is imparted to the piston, so that the temperature of the cross-sectionally angular portion (edge portion) excessively increases. This would cause inconveniences to occur which include cracks ascribed to a decrease in the high-temperature strength of the material of the piston, the thermal deformation thereof and a high thermal stress put thereon. Therefore, the durability of the combustion chamber decreases.
Under the circumstances, the applicant of the present invention proposed "PISTON COOLING STRUCTURE FOR DIESEL ENGINES" disclosed in Japanese Utility Model Laid-Open No. 10449/1995, with a view to reducing such inconveniences.
The invention disclosed in this publication is a piston cooling structure for diesel engines which is provided with a cooling cavity in an outer circumferential wall of a re-entrant type combustion chamber, the cooling cavity being provided with a cooling oil introduction port from which a cooling oil is injected thereinto, wherein an inner diameter of the portion of the cooling cavity which is on the side of a top portion of the piston is set smaller than that of the portion of the cooling cavity which is on the side of a lower portion of the piston.
Since the inner diameter of the portion of the cooling cavity which is on the side of the top surface of the piston is set smaller than that of the portion of the cooling cavity which is on the side of the lower portion of the piston, the cooling oil in the portion of the interior of the cooling cavity which is on the side of the top portion of the piston flows close to the aforementioned cross-sectionally angular portion (edge portion) as compared with the cooling oil in the corresponding portion of a cooling cavity of a piston of related techniques.
As a result, the efficiency of a cooling operation using a cooling oil can be improved, so that a piston having a thermal load resistance higher than that of a piston of related techniques could be obtained.
In the structure of a piston of related techniques, the inner diameter of the portion of the cooling cavity which is on the side of the top portion of the piston is set smaller than that of the portion of the cooling cavity which is on the side the lower portion of the piston, so that a cross-sectional area of the cooling cavity gradually increases from the lower side of the piston toward the top surface thereof. Therefore, it is hard to make this piston since it has so to speak a structure obtained by shaving off a circumferential wall thereof from the inner side of the cross-sectionally angular portion (edge portion) thereof.
The reasons why the wall of the piston is shaved off from the inner side of the cross-sectionally angular portion thereof are as follows. In the case of a re-entrant type combustion chamber, the air in a space (squished area) between a piston-top surface and an inner bottom surface of a cylinder is made to flow into a combustion chamber when a piston moves up, to cause a whirling current to occur in a deep recessed portion of the combustion chamber, whereby a fuel is atomized. Consequently, it is necessary that an edge of a cross- sectionally angular portion of the combustion chamber be left to as great an extent as possible as a part for generating a whirling current. Thus, in order to improve the combustion chamber cooling effect, it is necessary to form the combustion chamber by shaving off a wall thereof from the inner side of an edge section of the cross-sectionally angular portion thereof.
Moreover, even when the combustion chamber is formed in this manner by shaving off the wall thereof from the inner side of the edge section of the cross-sectionally angular portion thereof, the edge of the cross-sectionally angular portion still remains for the above-mentioned reason. Therefore, heat points readily occur on the edge, and a sufficiently much improvement of the durability of the piston cannot be expected. In addition, the difficulty in cooling the opposite portion of the combustion chamber, i.e. the lower portion thereof arises as a new problem.